1. Field of the Invention
The present invention relates to liquid crystal display devices, and more particularly, to a method for fabricating a liquid crystal display device using alignment keys.
2. Background of the Related Art
Keeping pace with the development of an information oriented society, demand for display devices in various forms have increased. To meet these demands, various types of flat display devices, such as an LCD (Liquid Crystal Display), PDP (Plasma Display Panel), ELD (Electro Luminescent Display) and VFD (Vacuum Fluorescent Display) devices, have been used as display devices in various apparatuses. Of the various types of display devices, the LCD is most widely used for mobile display devices due do its advantages of good picture quality, light weight, thin profile, and low power consumption. Besides mobile display devices, such as displays for notebook computers, the LCD has been used in televisions and as monitors for computers. Using the LCD in various fields as a general display device increase the need of a high quality picture, such as high definition, high luminance, and a large size picture, while maintaining the features of light weight, thin profile, and low power consumption. However, recent technical developments in LCD devices to serve as general display devices by enhancing the picture quality are contradictory to above advantages in many aspects.
FIG. 1 illustrates a disassembled perspective view of a part of a related art TN liquid crystal display device, including a lower substrate 1, and an upper substrate 2 bonded together with a space therebetween, a liquid crystal layer 3 between the lower substrate 1 and the upper substrate 2. The lower substrate 1 has a plurality of gate lines 4 arranged at regular intervals in one direction, and a plurality of data lines 5 arranged at regular intervals perpendicular to the gate lines 4, to define a plurality of pixel regions “P.” The upper substrate 2 has a black matrix layer 7 for shielding light to parts except the pixel regions ‘P’, RGB color filter layers 8 for displaying colors and a common electrode 9. The upper and lower substrates are bonded by a sealant. A space is maintained between the upper and lower substrates through the use of spacers.
A pixel electrode 6 and a thin film transistor “T” are located in each of the pixel regions. The thin film transistor “T” has a gate electrode projecting from the gate line 4, a gate insulating film (not shown) formed over an entire surface of the lower substrate 1, an active layer over the gate insulating film over the gate electrode, and a source electrode projected from the data line 5, and a drain electrode opposite to the source electrode. The pixel electrode 6 is formed of a transparent conductive metal having relatively high light transmitivity, such as indium-tin-oxide (ITO).
The twisted nematic (TN) type LCD can display a picture by orienting the liquid crystal layer 3 on the pixel electrode 6 using a signal applied through the thin film transistor “T” to manipulate the orientation of the liquid crystal layer 3 so as to control the quantity of light transmitting through the liquid crystal layer 3. The TN type LCD, which is driven by an electric field having an up/down direction, has the good characteristics of high light transmitivity and good aperture ratio. Further, a TN type LCD is resistant to static electricity because the common electrode 9 of the upper substrate 2 serves as ground.
The related art LCD has alignment problems. More particularly, the color filter layer on the upper layer is liable to misalign with the pixel region on the lower substrate during the bonding of the upper and the lower substrates. Such a positional deviation between the pixel regions on the lower substrate and the color filter layer on the upper substrate becomes an even greater problem when the substrates are larger and the apertures decrease (i.e. resolution increases). To solve these problems, a color filter On TFT (COT) array or TFT array On Color filter (TOC) structure has been proposed.
FIG. 2 illustrates a sectional view of a related art LCD of a COT structure and FIG. 3 illustrates a plan view of a related art LCD of a COT structure. Referring to FIG. 2, the related art LCD of a COT structure is provided with a lower substrate 20 having an active region thereon containing pixel regions for displaying a picture. RGB color filters 21 are positioned in each of the pixel regions within the active region on the lower substrate 20. A sealant 23 is positioned on a periphery of the active region of the lower substrate 20 to prevent leakage of the liquid crystals, and bonding the upper substrate 30 and lower substrate 20.
An alignment film 22 is formed over an entire surface of the lower substrate 20 and the RGB color filter layers 21. The alignment film 22 is formed larger than the active region with a preset margin “a” , and the sealant is spaced from the alignment film 22 with a preset margin “b” for preventing the alignment film 22 from peeling. Moreover, although not shown, there are alignment keys formed on the lower substrate 20 on an outer side of the sealant 23 formed at the time of patterning the gate lines and the data lines. There is also an alignment film 32 in an active region 32 of the upper substrate 30 shown in FIGS. 2 and 3.
To prevent back light leakage from outside of the active region, the upper substrate 30 has a light shielding film 31 on a periphery of the upper substrate 30, i.e., around the active region. Alignment keys 33 are formed on the upper substrate 30 and the lower substrate 20 on an outer side of the light shielding film 31 for an accurate alignment during different processes. The regions of the substrates that the alignment keys are formed on will be cut away after the bonding of the upper/lower substrates. Alignment keys 33 are, for an example, alignment film printing keys, bonding keys, and seal printing keys, and the like.
Since no black matrix layer is formed on the active region of the upper substrate, the COT structure may have a light shielding film of resin over a channel region of a thin film transistor on the lower substrate to prevent reflection of an external light. In the process for applying the resin over the channel region of the thin film transistor on the lower substrate, a light shielding film of resin is formed on the outer side of the active region of the lower substrate instead of on the outer side of the active region of the upper substrate. Although a structure is suggested, in which a light shielding film of resin is formed on an outer periphery of the active region of the lower substrate to shield a back light leaking from an outer part of the active region, even if the light shielding film is formed on the lower substrate thus, the fabrication process is not simple because it requires the formation of alignment keys on the upper substrate by a photo-etching process. That is, even if the light shielding film is formed on the outer side of the active region of the lower substrate instead of about the periphery of the active region of the upper substrate, there is a problem in that the process for forming the alignment keys on the outer side of the active region of the upper substrate 30 uses a photo-etching process, which is complicated and requires precision.